Telephone set circuit



June 9, 1942. DEN HERTQG I 2,286,069

TELEPHONESET CIRCUIT Filed NOV. 15, 1940 i, d RT X a Ml? Cord Ell/re g}?! 6 g R Y +14% F/GZ F/GZJ A; FT X d m ei: W K W 0/1/76 5 I? y 5 2 %6 T 2 2 FIGS aw w ATTO NE) Patented June 9, 1942 TELEPHONE SET CIRCUIT Martinus den Hertog, Antwerp, Belgium, assignor to Western Electric Company, Incorporated, New York, N. Y., a corporation of New York Application November 15, 1940, Serial No. 365,727 In Belgium December 9, 1939 6 Claims.

This invention relates to circuits for telephone subscribers sets.

It is known that non-linear resistance elements, such as copper oxide rectifiers, may be used to protect telephone circuits against high potentials causing clicks, by connecting two such dry rectifiers in opposite sense with respect to the direction in which direct current will pass, in parallel to the telephone circuit. The principle of the idea is that the resistance of the copper oxide rectifiers at low potentials is very high, so that they offer a very high resistance to the Voice frequency currents passing through the telephone circuit as the result of speech. With increasing potentials the resistance of the copper oxide rectifiers becomes less, and owing to this property, high potentials arriving on the line due to discharges or surges may find a path through a dry rectifier of comparatively low resistance, which thereby effectively prevents the listening party from receiving very loud clicks as the result of these high potentials.

From a consideration of this principle it follows that the dry rectifiers should be placed in a part of the circuit where, in the talking condition, there is no direct current potential, because the presence of such a potential would immediately lower the resistance of the rectifiers and thereby render them also conductive for voice frequency currents. When trying to apply the use of such dry rectifiers in subscriber sets, it is found that, under normal conditions, many subscriber sets are so wired that a direct current will flow through every part of the circuit, so that the rectifier cannot be connected anywhere without receiving a direct current bias.

The solution proposed for this problem is to connect the dry rectifiers in such a way in the subscriber set connections that it is connected to points of equal potential in a Wheatstone bridge of which the arms are constituted by certain components of the subscriber set. In order that the points between which the rectifiers are connected will have perfectly equal potential, it may be necessary to change the resistance of one or two of these arms so that the correct relation is obtained between the difierent arms. This may happen by adding a resistance in one arm or the other as may be found necessary, depending on the relative resistances of the different components, or it may be possible in certain cases also to connect the rectifiers to a tapping of one of the windings of the induction coils, in such a manner that the two divisions of this winding have the proper resistance so as to guarantee the correct relation between the difierent arms of the Wheatstone bridge.

In order that the invention may be clearly understood, a description will be given of certain embodiments thereof, reference being made for this purpose to the accompanying drawing, in which:

Fig. 1 shows the circuit of a telephone subscribers station;

Fig. 2 shows an equivalent circuit to that shown in Fig. 1, with the addition of a pair of rectifiers and balancing resistances in accordance With the present invention;

Fig. 3 show the same equivalent circuit as that shown in Fig. 2, but differently modified to incorporate the present invention;

Fig. 4 shows a second telephone subscriber's circuit; and

Fig. 5 shows the equivalent thereof in accordance with the present invention.

Fig. 1 shows in a simplified form the connections of one type of subscriber set when this is in the talking condition. For the sake of clarity the switchhook contacts and dial contacts have not been indicated. The figure also shows the addition of a pair of rectifiers MR; which are effectively connected in parallel to the receiver RC, and which have the purpose of taking up any high potentials which otherwise might reach the receiver. When the rectifier-pair is added in the manner shown, it will be seen that there is a small direct-current potential-difference across their terminals de, owing to the fact that direct current flows through the circuit in two ways as follows:

(a) From line terminal L1 through the primary winding P of the induction coil point (1, terminal RT, through conductor a: of the microtelephone cord, terminal a, transmitter TR, terminal c, conductor .2 of the microtelephone cord, terminal T, to line terminal L2.

(11) From line terminal L1, through ringer RI, secondary winding S of the induction coil point e, terminal R, conductor y of the microtelephone cord, terminal b receiver RC, transmitter TR, terminal 0, conductor a, terminal T, to line ter minal L2.

The risistances of P, x, RI, S, y and BC, in regular subscriber sets, are such that there is a certain direct-current potential-difierence between the terminals RT and R across which the rectifiers are connected, the points if and 6 being considered electrically coincident therewith.

lhe circuit of Fig. 1 has been represented again in Fig. 2 in a slightly different way, in-

dicating the components in the form of a Wheatstone bridge. It will be seen that the rectifierpair MR connected in this Wheatstone bridge will have no direct current potential across its terminals de when the resistance p of the arm including primary winding P, resistance q including line x, 1" including ringer RI and secondary S, and 3 including line 1/ and receiver RC, have the following relationship: ps=qr.

This condition can be met in different ways. Depending on the resistances of the different components, it may be possible to insert an additional resistance in either of the ways indicated by the resistances marked Ra and Rh. It may also be possible to add resistances in one of the other two branches of the Wheatstone bridge, but preference should be given to a scheme where the additional resistance affects the transmission and the sensitiveness of the ringer in the least possible manner. This will in most cases be obtained when the additional resistances are inserted in either of the branches 1) or q, i. e. in one of the two places indicated by Ra and Rb, because these branches have both a comparatively low resistance.

An alternative way of obtaining the conditions stipulated above is shown in Fig. 3 and consists in giving the winding P of the induction coil a middle tapping, to which the terminal (1 of the rectifier-pair is connected instead of to the terminal RT. The tapping should be made in such a way that the above stipulated condition is met, considering that one part of the coil P will now be inserted in branch 1), and the other part will be inserted in branch q. In this case it will not be necessary to add any additional resistances, so that the efficiency of the set will not be reduced by such addition.

Fig. shows in a simplified manner the talking condition of a modified circuit for a subscriber set, in which the induction coil is provided with a third winding Te, for the purpose of reducing the side-tone eifect; and Fig. 5 shows the equivalent Wheatstones bridge circuit to which has been added a rectifierepair MR together with a resistance either at the place marked Ra or "Rb, so as to obtain a condition in which no directcurrent potential exists across the terminals of the rectifiers. It will be seen that when the four branches of the Wheatstone bridge are so balanced that no direct current exists on the terminals of the rectifier, the tertiary winding Te has no direct-current potential either, and from a point of view of distribution of direct current, the circuit becomes exactly identical to the circuit of Figs. 1 and 2.

What is claimed is:

1. A telephone substation circuit comprising a pair of line conductor terminals, a transmitter, a receiver, an induction coil, a ringer, and a receiver click-reduction device, said receiver and the windings of said induction coil and said ringer being connected to form a Wheatstone bridge, said transmitter and said bridge being seriallyconnected across said line terminals, one of the arms of said bridge comprising said receiver, another arm comprising one winding of said induction coil, a third arm comprising said ringer and a second winding of the induction coil, and a fourth arm comprising a circuit connection between said one induction coil winding and said receiver and transmitter, the click-reduction device being connected between such points on said bridge that, while said circuit is being used, the direct current flowing in the circuit is not effective on said device.

2. A telephone substation circuit as claimed in claim 1 in which an auxiliary resistance is included in one or more arms of the bridge to establish the balance required if the circuit components are not of appropriate resistances.

3. A telephone substation circuit as claimed in claim 1 in which one'of the points to which the click-reduction device is connected is a tap on one winding of the induction coil.

4. A telephone set circuit comprising a pair of line conductor terminals, a serially connected transmitter and Wheatstone bridge across said line terminals, said Wheatstone bridge comprising a two-winding induction coil, a ringer, a receiver, a receiver click-reduction device and one or more resistances, said receiver constituting one arm of the bridge,'one induction coil winding a second arm, a resistance a third arm, and serially-connected ringer and the second induction coil winding the fourth arm, said click-reduction device being connected between the junctions of said one and fourth arms and said second and third arms, and a condenser connected between one terminal and the junction of said ringer and second induction coil winding.

5. A telephone set circuit comprising a pair of line conductor terminals, a serially-connected transmitted and Wheatstone bridge across said line terminals, said Wheatstone bridge comprising a two=winding induction coil, a ringer, a receiver, and a receiver click-reduction device, said receiver constituting one arm of the bridge, a portion of one induction coil winding 2. second arm, another portion of said one induction coil winding a third arm, and serially-connected ringer and the other induction coil winding the fourth arm, said click-reduction device being connected between the junctions of said one and fourth arms and said second and third arms, and a condenser connected between one line terminal and the junction of said ringer and second induction coil winding.

6. A telephone set circuit comprising a pair of line conductor terminals, a serially-connected transmitter and Wheatstone bridge across said line terminals, said Wheatstone bridge comprising a multiwinding induction coil, a ringer, a receiver, a receiver click-reduction device and one or more resistances, said receiver constituting one arm of the bridge, one induction coil winding a second arm, a resistanceathird arm, and seriallyconnected ringer and a second induction coil winding the fourth arm, said click-reduction receiver and a third induction coil winding being connected between the junctions of said one and fourth arms and said second and third arms, and a condenser connected between one line terminal and the junction of said ringer and second induction coil winding.

MARTINUS DEN HERTOG. 

